fix(sidebar): unify sections by declaring them with CSS classes

[caugner]

Summary

Fixes #9917.

Problem

1. Some sidebars contain unlinked section titles, and these were positioned differently than linked section titles.
2. Linked section titles were inconsistently created by nesting <a> and <strong> within each other.
3. Overall, the spacing of section titles was inconsistent.

Solution

1. Use the existing CSS class no-link to add missing padding to unlinked section titles (all links in the sidebar already have additional padding).
2. Add the CSS class section for sidebar section titles and remove <strong> tags.
3. Set margin-top: 1.5rem for all section titles (increases spacing for most, decreases spacing slightly for some).

Note: This has the (positive?) side-effect that section titles are now also highlighted if they are the current page.

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Screenshots

Macro	Before	After
CSSRef
HTMLSidebar
HTTPSidebar
JSRef
MathMLRef
SVGRef

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How did you test this change?

Ran yarn && yarn dev locally and looked at the following pages:

• http://localhost:3000/en-US/docs/Web/CSS
• http://localhost:3000/en-US/docs/Web/HTML
• http://localhost:3000/en-US/docs/Web/HTTP
• http://localhost:3000/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator
• http://localhost:3000/en-US/docs/Web/MathML
• http://localhost:3000/en-US/docs/Web/SVG

[caugner]

FYI @argl @fiji-flo I checked and no new occurrences of <strong> have been introduced in sidebars since creating this PR.

[argl]

Looks good, love the rigorous combing through the sidebar markup, making all a bit simpler.